An LDD disc is a video disc in which an audio signal is digitized through a predetermined digital modulation approach into a train of pulse signals and is converted into an FM modulated signal for recording together with an FM modulated video signal and an FM modulated analog audio signal.
With the LDD disc, the audio signal is split into two channels where audio carriers of 2.3 MHz and 2.8 MHz, respectively, are frequency modulated by the two-channel audio signals. The video signal is frequency modulated so that a sync tip is at 7.6 MHz, a pedestal level is at 8.1 MHz, and a white peak is at 9.3 MHz, respectively. The audio signal is digitized through, for example, Pulse Code Modulation (PCM) into a train of pulse signals. The pulse train signal is suitable for recording through the Eight-to-Fourteen Modulation (EFM) technique and has frequency components of a train of pulses having widths of 3T-11T, where T represents a bit period of the PCM signal. A pulse of 3T is about 720 KHz and 11T is about 200 KHz, which is the maximum. The pulse train signal is superimposed at a level of less than about 1/10 of that of the main video carrier, and is slice-amplified at levels in the vicinity of zero-crossing points to be converted into a pulse modulated signal for recording.
FIG. 6 shows the frequency spectrum of an RF signal reproduced from the LDD disc of the above-described recording method. Range A represents the spectrum of a digital stereo audio signal, B shows the spectrum of an audio FM signal of the left and right channels, and C shows the spectrum of a video FM signal.
With such LDD discs, particularly so-called KARAOKE (i.e., sing-along) discs, one of the analog audio signals recorded in the two channels contains a monaural accompaniment while the other channel includes the vocal part in addition to the monaural accompaniment. On the other hand, the digital stereo audio signal contains only the stereo accompaniment. Therefore, when one wishes to listen to the vocal part, the conventional disc player suffers from a problem that there is not the same feeling of listening to a live orchestra due to the fact that the accompaniment is reproduced using only the monaural mode of the analog audio signal reproduced from the disc.